The measurement of the dielectric constant of a formation surrounding a borehole can be used to determine water saturation, for example, if the lithology and porosity of that formation have been determined. One logging device for measuring the dielectric constant of a formation is the electromagnetic propagation tool (EPT, a mark of Schlumberger). The EPT measures the propagation time and attenuation of an electromagnetic wave that has been propagated through the formation near the borehole. The EPT has been described in U.S. Pat. No. 3,944,910, for example. This patent has been assigned to the same assignee as the present invention, and the disclosure of this patent is incorporated by reference.
The EPT has an antenna pad that is rigidly attached to the body of the tool. A back-up arm opens from the tool and forces the antenna pad against the borehole wall. The back-up arm also provides a caliper measurement of the borehole wall. The antenna pad includes two microwave transmitters and two receivers which are mounted in a borehole-compensated array. This arrangement minimizes the effects of borehole rugosity and tool tilt.
U.S. Pat. No. 3,944,910 describes such transmitters and receivers, which comprise cavity-backed slot antennas. Each cavity-backed slot antenna is filled with a dielectric material and includes a probe. The probe extends across the cavity-backed antenna parallel to the longitudinal axis of the logging device.
An adaptable EPT (ADEPT) provides superior measurement of the dielectric constant in rugose boreholes and in the presence of mudcake. The ADEPT uses one of two optional antenna arrays known as the endfire array or the broadside array. The two antenna arrays are mounted separately on exchangeable antenna pads. The ADEPT can be configured according to expected logging by a quick change of antenna pads. In fact, if saltier muds of formation waters are unexpectedly encountered during a logging pass of the ADEPT using the endfire array, the antenna pads can be changed on-site, and a repeat pass can be made using the broadside array. Neither antenna array produces an electronically adjustable magnetic moment.
FIG. 1a is a schematic diagram of an ADEPT having a pad that carries an endfire array 10. Two microwave receivers 12 are located between two microwave transmitters 14 with a spacing of 80 mm and 120 mm. The two receivers 12 and the two transmitters 14 are oriented vertically on the ADEPT, parallel to the longitudinal axis of the ADEPT and the borehole. Each receiver 12 and transmitter 14 comprises a rectangular slot antenna 16 having one probe element 18 perpendicular to the ADEPT and borehole axis. The endfire array 10 is less affected by hole roughness and tool standoff from mudcake than a conventional logging antenna system. The endfire array 10 has a good depth of investigation without sacrificing vertical resolution. The endfire array 10 is normally used when invaded zone resistivity is greater than 1 ohm-m.
FIG. 1B is a schematic diagram of an ADEPT having a pad that carries a broadside array 20. Two microwave receivers 22 are located between two microwave transmitters 24 with a spacing of 40 mm and 80 mm. The two receivers 22 and the two transmitters 24 are oriented horizontally on the ADEPT, perpendicular to the longitudinal axis of the ADEPT and the borehole. Each receiver 22 and transmitter 24 comprises a rectangular slot antenna 26 having one probe element 28 parallel to the ADEPT and borehole axis. The short spacing between the receivers 22 and the transmitters 24 extends the effective operating range in high porosity or saline conditions, which increase attenuation of signals between such receivers and transmitters. The broadside array 20 has less depth of investigation than the endfire array 20, but has a sharper image response. The broadside array 20 is normally used in the ADEPT when invaded zone resistivity is less than 1 ohm/m.
FIGS. 2a and 2b illustrate the simulated standoff response of the endfire array 10 of FIG. 1a and the broadside array 20 of FIG. 1B at 1.1 GHz. For FIGS. 2a and 2b, formation conductivity is 0.9 S/m, which is Seimans/meter, an equivalent of Mhos/meter, and formation permittivity is 22. Standoff layer conductivity is 4.0 S/m and standoff layer permittivity is 73. FIG. 2a illustrates inverted permittivity as a function of standoff of the array, endfire or broadside, from the borehole wall in inches. FIG. 2b illustrates inverted conductivity in mho/m as a function of standoff in inches. The endfire array of FIG. 1a has better standoff performance than the broadside array. In FIGS. 2a and 2b, a solid, horizontal line indicates ideal standoff response. FIGS. 2a and 2b show that the standoff response of the endfire array is more nearly horizontal, and thus more closely approximates the ideal case than the standoff response of the broadside array. Standoff performance of the endfire array is typically good for standoff distances to 0.25".
U.S. Pat. No. 4,704,581, also assigned to the same assignee as this invention and incorporated by reference, describes a logging device having transmitters and receivers that comprise slot antennas. Each slot antenna includes a probe that extends across the cavity-backed antenna parallel to the longitudinal axis of the logging device in one embodiment, or extends across the cavity-backed antenna perpendicular to the longitudinal axis of the logging device in another embodiment.
None of the above devices concerns a slot antenna having two elements which are energized to produce an electronically adjustable angle of magnetic moment.